This invention relates to an electrified object contact component.
In an LSI manufacturing process, electrification of a wafer is a big problem, and it is urgently required to establish a technology for prevention of electrification.
Description is hereunder made for troubles caused by electrification of a wafer, as an example of an electrified object.
As a wafer is generally handled with insulating fluororesin or quartz for preventing it from being contaminated, a very high electric potential is charged when the wafer contacts a contacting member while being handled. Results of measurement of electric potential in electrified wafers are shown in Table 1 below.
TABLE 1 ______________________________________ Electric potential in an electrified wafer (23 degrees Centigrade, 30-45%) ______________________________________ When handled by a teflon forceps . . . +500 V.about.+3300 V or more When put on a polypropyrene stand . . . +600 V.about.+2000 V When a wafer is put on a quartz plate with a teflon forceps . . . +1000 V.about.+1500 V ______________________________________
A range of measurement by an electrometer=-3300 V.about.+3300 V
As shown by this result, it has turned out that, when a silicon wafer is handled by a resin material or quartz, always positive electricity is charged in the wafer because of the electrification column, and also that the electric potential is fairly high.
Also it has turned out that, when a wafer is electrified, the following two types of problems occur and cause substantial decrease in yield in a semiconductor manufacturing process;
1 Adhesion of airborne particles due to static electricity force
2 Destruction of a device due to discharge of static electricity
Results of a testing to investigate the problem 1 and computing results for the trouble are introduced below. FIG. 1 shows a number of particles with a diameter of 0.5 .mu.m or more which adhered to a surface of an electrified wafer when a 5-inch wafer was left on a conductive grating floor for 5 to 10 hours in a clean room in a vertical position with a 2 cm high insulating stand. The horizontal axis shows electric potentials in the wafer and the vertical axis shows a number of deposited particles (converted to a number of particles which adhered to a central area of a wafer when the wafer is left for 5 hours in the atmosphere with a density of 10 particles having a diameter of 0.5 .mu.m or more/cf). As adhesion of particles due to gravity does not occur on a vertical surface, adhesion of particles is not observed when electric potential of a wafer is in a low range from 0 V to 50 V. In accordance with increase of electric potential of the wafer to 300 V or to 1800 V, the number of adhered particles sharply increases, which shows that the adhesion is caused by a static electricity force. FIG. 1 shows results of measurement of effects by static electricity force to relatively large particles, and generally as diameter of a particle becomes smaller, effects of this static electricity force become more visible acceleratingly. When electric potential of a wafer is at least below 50 V, any particle does not adhere to the wafer. Herein, a state where electric potential of a wafer is 50 V or below is defined as a state where electric potential of the wafer has been neutralized. FIG. 2 shows a range of movement of particles moved and adhered due to static electricity force on an effective section of a wafer calculated on the assumption that electric potential of the wafer is 1000 V and electric potential at the peripheral rectangular frame line is zero. As a force to make particles adhere to a wafer, only gravity (including buoyancy) and static electricity force are taken into account. Also it is assumed that the particle density is 1 g/cm.sup.3. This drawing shows that particles in an area enclosed by oblique lines adhere to the effective section of the wafer. Results of the calculation show that an area where particles with the diameter of 2 .mu.m or more adhere to is very narrow and virtually no particle adheres to the wafer. As a particle diameter becomes smaller to 0.5 .mu.m or to 0.1 .mu.m, the adhesion area sharply becomes larger, which indicates that, when diameter of a particle is small, effect of static electricity force over the particle in terms of adhesion to a surface of an object is very large.
Results of the experiment and calculation described above indicate that preventing a wafer from being electrified is very important for preventing a surface of the wafer from being contaminated by particles.